2,350 research outputs found
Cyclotron Resonance Study of the Two-Dimensional Electron Layers and Double-Layers in Tilted Magnetic Fields
The far-infrared absorption in two-dimensional electron layers subject to
magnetic field of general orientation was studied theoretically. The Kubo
formula is employed to derive diagonal components of the magneto-conductivity
tensor of two-dimensional electron single-layers and double-layers. The
parabolic quantum well is used to model a simple single-layer system. Both
single-layer and double-layer systems can be realized in a pair of
tunnel-coupled, strictly two-dimensional quantum wells. Obtained results are
compared to experimental data.Comment: 4 pages, 6 figures, elsart/PHYEAUTH macros; presented on the EP2DS-15
Conference in Nara, Japan. To be published in Physica
Bose-Einstein Condensation of Erbium
We report on the achievement of Bose-Einstein condensation of erbium atoms
and on the observation of magnetic Feshbach resonances at low magnetic field.
By means of evaporative cooling in an optical dipole trap, we produce pure
condensates of Er, containing up to atoms. Feshbach
spectroscopy reveals an extraordinary rich loss spectrum with six loss
resonances already in a narrow magnetic-field range up to 3 G. Finally, we
demonstrate the application of a low-field Feshbach resonance to produce a
tunable dipolar Bose-Einstein condensate and we observe its characteristic
d-wave collapse.Comment: 4 pages, 3 figure
Evidence for DCO+ as a probe of ionization in the warm disk surface
In this Letter we model the chemistry of DCO in protoplanetary disks.
We find that the overall distribution of the DCO abundance is
qualitatively similar to that of CO but is dominated by thin layer located at
the inner disk surface. To understand its distribution, we investigate the
different key gas-phase deuteration pathways that can lead to the formation of
DCO. Our analysis shows that the recent update in the exothermicity of
the reaction involving CHD as a parent molecule of DCO favors
deuterium fractionation in warmer conditions. As a result the formation of
DCO is enhanced in the inner warm surface layers of the disk where X-ray
ionization occurs. Our analysis points out that DCO is not a reliable
tracer of the CO snow line as previously suggested. We thus predict that
DCO is a tracer of active deuterium and in particular X-ray ionization of
the inner disk.Comment: Accepted for publication in the Astrophysical Journal Letters (ApJL).
11 pages, 5 figure
Resolving the chemistry in the disk of TW Hydrae I. Deuterated species
We present Submillimeter Array (SMA) observations of several deuterated
species in the disk around the classical T Tauri star TW Hydrae at arcsecond
scales, including detections of the DCN J=3-2 and DCO+ J=3-2 lines, and upper
limits to the HDO 3(1,2)-2(2,1), ortho-H2D+ 1(1,0)-1(1,1) and para-D2H+
1(1,0)-1(0,1) transitions. We also present observations of the HCN J=3-2, HCO+
J=3-2 and H13CO+ J=4-3 lines for comparison with their deuterated
isotopologues. We constrain the radial and vertical distributions of various
species in the disk by fitting the data using a model where the molecular
emission from an irradiated accretion disk is sampled with a 2D Monte Carlo
radiative transfer code. We find that the distribution of DCO+ differs markedly
from that of HCO+. The D/H ratios inferred change by at least one order of
magnitude (0.01 to 0.1) for radii 70 AU and there is a rapid falloff
of the abundance of DCO+ at radii larger than 90 AU. Using a simple analytical
chemical model, we constrain the degree of ionization, x(e-)=n(e-)/n(H2), to be
~10^-7 in the disk layer(s) where these molecules are present. Provided the
distribution of DCN follows that of HCN, the ratio of DCN to HCN is determined
to be 1.7\pm0.5 \times 10^-2; however, this ratio is very sensitive to the
poorly constrained vertical distribution of HCN. The resolved radial
distribution of DCO+ indicates that {\it in situ} deuterium fractionation
remains active within the TW Hydrae disk and must be considered in the
molecular evolution of circumstellar accretion disks.Comment: 12 pages, 12 figures, accepted to Ap
Cold CO Gas in Protoplanetary Disks
In a disk around DM Tau, previous observation of 13CO (J=2-1 and 1-0
transitions) derived the 13CO gas temperature of \sim 13-20K, which is lower
than the sublimation temperature of CO (20 K). We argue that the existence of
such cold CO can be explained by a vertical mixing of disk material. As the gas
is transported from a warm layer to a cold layer, CO is depleted onto dust
grains with a timescale of \sim 10^3 yr. Because of the steep temperature
gradient in the vertical direction, an observable amount of CO is still in the
gas phase when the fluid parcel reaches the layer of \sim 13 K. Apparent
temperature of CO decreases as the maximum grain size increases from
micron-size to mm-size.Comment: 11 pages, 2 figures, accepted to ApJ
Fano Resonance in a Quantum Wire with a Side-coupled Quantum Dot
We report a transport experiment on the Fano effect in a quantum connecting
wire (QW) with a side-coupled quantum dot (QD). The Fano resonance occurs
between the QD and the "T-shaped" junction in the wire, and the transport
detects anti-resonance or forward scattered part of the wavefunction. While in
this geometry it is more difficult to tune the shape of the resonance than in
the previously reported Aharonov-Bohm-ring type interferometer, the resonance
purely consists of the coherent part of transport. By utilizing this advantage,
we have qualitatively explained the temperature dependence of the Fano effect
by including the thermal broadening and the decoherence. We have also proven
that this geometry can be a useful interferometer to measure the phase
evolution of electrons at a QD.Comment: REVTEX, 6 pages including 5 figures, final versio
Narrow-line magneto-optical trap for erbium
We report on the experimental realization of a robust and efficient
magneto-optical trap for erbium atoms, based on a narrow cooling transition at
583nm. We observe up to atoms at a temperature of about
. This simple scheme provides better starting conditions for direct
loading of dipole traps as compared to approaches based on the strong cooling
transition alone, or on a combination of a strong and a narrow kHz transition.
Our results on Er point to a general, simple and efficient approach to laser
cool samples of other lanthanide atoms (Ho, Dy, and Tm) for the production of
quantum-degenerate samples
Breakdown of `phase rigidity' and variations of the Fano effect in closed Aharonov-Bohm interferometers
Although the conductance of a closed Aharonov-Bohm interferometer, with a
quantum dot on one branch, obeys the Onsager symmetry under magnetic field
reversal, it needs not be a periodic function of this field: the conductance
maxima move with both the field and the gate voltage on the dot, in an apparent
breakdown of `phase rigidity'. These experimental findings are explained
theoretically as resulting from multiple electronic paths around the
interferometer ring. Data containing several Coulomb blockade peaks, whose
shapes change with the magnetic flux, are fitted to a simple model, in which
each resonant level on the dot couples to a different path around the ring
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